The manufacture of oil seal rings of elastomeric material is fraught with difficulties. Among the many challenges is to create a seal whose sealing surface is within certain critical dimensions, usually in terms of a nominal diameter plus or minus a tolerance, usually expressed in .001".
There have been several methods available to test oil seals to determine the actual diameter of the sealing surface. The simplest, but one that is quite time consuming, is the use of a gage shaft (also known as a "step pin") on which incremental steps of known diameters have been machined. A skilled operator can judge which of the stepped diameters most closely approximates the actual i.d. (inside diameter) of the seal somewhat by the "feel" of the shaft inserted in the oil seal being tested, sensing when there was just the right drag or resistance to sliding between step pin and the seal.
This "feel" test is sometimes supplimented by the use of a light box. In this procedure, the operator attempts to view the light box through any gap between the gage rod and the seal. This also checks for any nicks in the oil seal lip as well as the overall dimension of the seal.
These techniques require considerable experience and training. Repeatability is affected by a number of factors including the friction characteristics of the rubber compound used in the seal and whether the step pin or the seal lip were contaminated with lubricants, mold release compounds etc.
These methods are also cumbersome because of the need for supplying large numbers of the bulky step pins. Also, the pins necessarily have only a finite number of incremental diameters represented, and thus the precision of the test is limited to the closeness to which the nominal diameter of the seal can be approximately by the diameter of the closest available step on the gage rod.
Also, the subjective nature of the test limits its accuracy and repeatability.
Another known method includes the use of an optical gage that employs a microscope on a traverse to detect the inner edge of a seal. The diameter of the seal is determined by the precise measurement of the position of the microscope when the image of the edge is detected at a particular position in its field of view. Such instruments can include automatic feedback control devices to detect the image of the edge.
This method has the advantage that it does not contact the extremely flexible edge of the seal, which contact could deflect that edge and thus result in inaccurate measurements. However, repeatability is still a problem. Also, the sealing surface rarely presents a truly circular dimension, so a number of readings must be made in order to determine the i.d. by averaging these readings.